1. Field of the Invention
The present invention generally relates to implements used to hold or support articles being heated in an oven, and more particularly to a form or holder which retains a coiled optical fiber during an annealing process, and to the subsequently produced coil subassembly for a Faraday-effect magnetic field sensor.
2. Description of the Prior Art
Optical fibers which are sensitive to magnetic fields are known in the art, and are increasingly being used as optical current transducers (OCT's) for electric power utilities. A typical OCT uses a single-mode fiber formed into a coil surrounding the electrical conductor. The polarization of any light traversing the fiber coil shifts, in response to the change in any current flowing through the conductor, as a result of the magneto-optic "Faraday" effect. Further discussion of field-sensitive optical fibers is provided in U.S. Pat. No. 5,051,577 assigned to Minnesota Mining and Manufacturing Co. (3M-assignee of the present invention).
An optical medium becomes less suitable for use in a Faraday-effect sensor as its linear birefringence increases, which makes the coil more sensitive to external magnetic fields and distorts the polarized light signal. Care is particularly required in fiber selection since the formation of loops of a fiber into a coil introduces physical stresses which may further increase birefringence. One conventional method of reducing linear birefringence relies upon relieving or eliminating internal stresses present in the fiber coil. Such stresses may be produced by bending forces or transverse pressure applied to the fiber during manufacture, as well as from the stress induced when the coil is formed. Improvement in the performance of field-sensing optical fiber coils is consequently possible by annealing the coils at a temperature at which stress relaxation occurs.
A convenient implement used in annealing fiber coils may be constructed from a machinable ceramic material such as MACOR, sold by Corning, and consists of a ceramic plate having a pair of channels machined or molded in the upper surface, leading to a circular groove, as described in the literature (e.g., vol. 9, J. Lightwave Tech. pp. 1031-1037). The groove forms a cylindrical wall about which the fiber may be loosely wound. After the fiber coil has been annealed, it is removed from the plate and placed in the sensor assembly. While the surface of this plate may be precisely machined, the use of a ceramic plate presents several problems. First of all, it limits the temperature of the annealing cycle to around 850.degree. C., well below the usual annealing temperature for pure silica, which comprises the bulk of an optical fiber. This lower temperature requires 50-100 hours to sufficiently anneal the fibers, resulting in a slow process with high energy cost. A second problem is that the plate is very fragile and easily broken, particularly if it is repeatedly subjected to heating up to temperatures of 800.degree. C. or more. Also, the large differential between thermal expansion coefficients of the ceramic material and silica optical fibers can lead to breakage of the fibers during annealing. Silica holders have not been used, perhaps due to the difficultly in machining such material. A final problem is that it is difficult to remove the fiber coil and transport it to the sensor assembly without damaging it or inducing stresses on the fiber which can increase birefringence. It would, therefore, be desirable to devise a holder for a fiber optic coil which would overcome the limitations of ceramic holders, and yet still not be difficult to fashion into an appropriate shape. It would be further advantageous if the holder design would avoid or minimize any damage to the fiber coil during fabrication of the sensor subassembly.